FIGS. 1A, 1B and 1C show a prior art transmission device for a vertical blind. As illustrated, the transmission device includes a housing A in which are fitted a worm B and a worm gear C. The worm B is provided with a single tooth B1 adapted to engage the worm gear C. Further, the worm gear C has a partition C1 for engaging the end of the tooth B1 of the worm B so as to limit the worm gear C to rotate through a maximum angle of 180 degrees only. However, as a twisting force is exerted on the vane D, the worm gear C will be urged to skip onto the tooth of the worm B.
Hence, another prior art transmission device (see FIGS. 2A, 2B and 2C) for a vertical blind has been developed to obviate this drawback. As can be seen, the worm B is provided with a plurality of teeth B1 so that at least two teeth B1 of the worm B will be engaged with the worm gear C simultaneously. In addition, the housing A is formed with a neck portion A1 having a notch A2 for engaging a protuberance C2 of the worm gear C thereby limiting the worm gear C to rotate through a maximum angle of 180 degrees. Nevertheless, it is not easy to feel that the protuberance C2 of the worm gear C has been blocked by the notch A2 of the housing thereby often excessively rotating the worm B and therefore breaking the housing A.
Therefore, it is an object of the present invention to provide an improved transmission device for a vertical blind which can obviate and mitigate the above-mentioned drawbacks.